sedimentary

Ironstone Concretion

Siderite or Limonite Concretion (FeCO3 or FeO(OH)·nH2O)

AI-generated identification · may be incorrect

Hardness: 4.0-5.5 (Mohs scale); Color: Rusty brown, orange, or dark gray; Luster: Dull or earthy; Structure: Nodular, often with concentric layers; Specific Gravity: 3.0-4.0.

Hardness: 4
Color: Rusty brown, orange, or dark gray
Luster: Dull or earthy

Identified June 12, 2026More sedimentary →

Explore Ironstone Concretion in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Hardness: 4.0-5.5 (Mohs scale); Color: Rusty brown, orange, or dark gray; Luster: Dull or earthy; Structure: Nodular, often with concentric layers; Specific Gravity: 3.0-4.0.

Formation & geological history

Formed by the precipitation of mineral cement (iron oxides or carbonates) around a nucleus, such as a fossil or organic fragment, within sedimentary layers like shale or sandstone. Common in Cretaceous and Paleozoic deposits.

Uses & applications

Used historically as low-grade iron ore; occasionally used in landscaping or as curiosities for fossil and mineral collectors.

Geological facts

Mazon Creek in Illinois is famous for concretions that split open to reveal exceptionally preserved soft-bodied fossils. Some concretions are mistaken for meteorites or dinosaur eggs due to their round shapes.

Field identification & locations

Identify in the field by their rounded or disc-like shape, high weight relative to size (high density), and reddish-brown weathering. Often found in riverbeds or eroding shale cliffs.